Gas turbine engines are employed in a variety of applications including electric power generation, military and commercial aviation, pipeline transmission and marine transportation. In a gas turbine engine which operates in LPP mode, fuel and air are provided to a burner chamber where they are mixed and ignited by a flame, thereby initiating combustion. The major problems associated with the combustion process in gas turbine engines, in addition to thermal efficiency and proper mixing of the fuel and the air, are associated to flame stabilization, the elimination of pulsations and noise, and the control of polluting emissions, especially nitrogen oxides (NOx), CO, UHC, smoke and particulated emission.
U.S. Pat. No. 6,152,724 A and EP 1 710 504 A2 respectively disclose a burner comprising swirler wings and fuel injectors to provide a mixture of fuel and air to a combustion chamber with a specific fuel and velocity distribution.
In industrial gas turbine engines, which operate in LPP mode, flame temperature is reduced by an addition of more air than required for the combustion process itself. The excess air that is not reacted must be heated during combustion, and as a result flame temperature of the combustion process is reduced (below stoichiometric point) from approximately 2300K to 1800 K and below. This reduction in flame temperature is required in order to significantly reduce NOx emissions. A method shown to be most successful in reducing NOx emissions is to make combustion process so lean that the temperature of the flame is reduced below the temperature at which diatomic Nitrogen and Oxygen (N2 and O2) dissociate and recombine into NO and NO2. Swirl stabilized combustion flows are commonly used in industrial gas turbine engines to stabilize combustion by, as indicated above, developing reverse flow (Swirl Induced Recirculation Zone) about the centreline, whereby the reverse flow returns heat and free radicals back to the incoming un-burnt fuel and air mixture. The heat and free radicals from the previously reacted fuel and air are required to initiate (pyrolyze fuel and initiate chain branching process) and sustain stable combustion of the fresh un-reacted fuel and air mixture. Stable combustion in gas turbine engines requires a cyclic process of combustion producing combustion products that are transported back upstream to initiate the combustion process. A flame front is stabilised in a Shear-Layer of the Swirl Induced Recirculation Zone. Within the Shear-Layer “Local Turbulent Flame Speed of the Air/Fuel Mixture” has to be higher then “Local Air/Fuel Mixture Velocity” and as a result the Flame Front/combustion process can be stabilised.
Lean premixed combustion is inherently less stable than diffusion flame combustion for the following reasons:
The amount of air required to reduce the flame temperature from 2300K to 1700-1800 K is approximately twice the amount of air required for stoichiometric combustion. This makes the overall fuel/air ratio (Φ) very close (around or below 0.5; Φ≧0.5) or similar to a fuel/air ratio at which lean extinction of the premixed flame occurs. Under these conditions the flame can locally extinguish and re-light in a periodic manner.
Near the lean extinction limit the flame speed of the lean partially premixed flames is very sensitive to the equivalence ratio fluctuations. Fluctuations in flame speed can result in spatial fluctuations/movements of the flame front (Swirl Induced Recirculation Zone). A less stable, easy to move flame front of a pre-mixed flame results in a periodic heat release rate, that, in turn, results in movement of the flame, unsteady fluid dynamic processes, and thermo-acoustic instabilities develop.
Equivalence ratio fluctuations are probably the most common coupling mechanism to link unsteady heat release to unsteady pressure oscillations.
In order to make the combustion sufficiently lean, in order to be able to significantly reduce NOx emissions, nearly all of the air used in the engine must go through the injector and has to be premixed with fuel. Therefore, all the flow in the burners has the potential to be reactive and requires that the point where combustion is initiated is fixed.
When the heat required for reactions to occur is the stability-limiting factor, very small temporal fluctuations in fuel/air equivalence ratios (which could either result either from fluctuation of fuel or air flow through the Burner/Injector) can cause flame to partially extinguish and re-light.
An additional and very important reason for the decrease in stability in the pre-mixed flame is that the steep gradient of fuel and air mixing is eliminated from the combustion process. This makes the premixed flow combustible anywhere where there is a sufficient temperature for reaction to occur. When the flame can, more easily, occur in multiple positions, it becomes more unstable. The only means for stabilizing a premixed flame to a fixed position are based on the temperature gradient produced where the unburnt premixed fuel and air mix with the hot products of combustion (flame cannot occur where the temperature is too low). This leaves the thermal gradient produced by the generation, radiation, diffusion and convection of heat as a method to stabilize the premixed flame. Radiation heating of the fluid does not produce a sharp gradient; therefore, stability must come from the generation, diffusion and convection of heat into the pre-reacted zone. Diffusion only produces a sharp gradient in laminar flow and not turbulent flows, leaving only convection and energy generation to produce the sharp gradients desired for flame stabilization which is actually heat and free radial gradients. Both, heat and free radial gradients, are generated, diffused and convected by the same mechanisms through recirculating products of combustion within the Swirl Induced Recirculation Zone.
In pre-mixed flows, as well as diffusion flows, rapid expansion causing separations and swirling recirculating flows, are both commonly used to produce gradients of heat and free radicals into the pre-reacted fuel and air.